1. Field of the Invention
The present invention relates to novel, liquid, non-flammable dielectric compositions, and more especially, to such dielectric compositions well suited as insulators/coolants for electrical transformers and the like.
2. Description of the Prior Art
It is well known to this art that the dielectric liquids utilized in electrical apparatus, and especially in transformers, perform a dual function. On the one hand, they serve as insulating liquids, and in this respect they must satisfy certain requirements concerning their electrical properties, in particular their dielectric strength and coefficient of dissipation. On the other hand, they concurrently serve as a cooling agent for the apparatus, in which case they must ensure excellent removal or dissipation of the heat generated during its operation. This latter function can only be successfully fulfilled if the agent employed possesses a sufficiently low viscosity, under the very variable conditions of use of such apparatus, for the liquid to readily dissipate the heat evolved. It is also art recognized that transformers, for example, may be required to function at extremely low external temperatures, for example, temperatures which are below 0.degree. C. and even as low as -40.degree. C. It is therefore important that, at these extreme temperatures, the dielectric remains a liquid having adequate fluidity and, moreover, does not give rise to complete crystallization. Consequently, same must remain liquid, or at least partially liquid, so that the crystals which would develop during prolonged periods at low temperatures would always be impregnated and surrounded by liquid, which makes it possible to prevent air from penetrating into the working parts of the transformer.
In addition to these properties, the dielectric liquids for certain types of apparatus are also required to be non-flammable. In fact, under the operating conditions of such apparatus (for example, transformers), a destruction of the dielectric can occur upon the formation of an electric arc which is indeed quite powerful. This breakdown arc decomposes the liquid or solid dielectrics and can ignite the liquid and/or the gases evolved, whether these are decomposition products of the dielectric or of the vapors thereof. It is thus important that the dielectric liquid and its vapors, or the decomposition gases produced in the event of a malfunction in the apparatus, do not ignite. In general, this ignition resistance is assessed in terms of the flash point of the liquid in question and of the results of certain fire resistance tests.
Numerous liquid dielectrics, possessing, to a greater or lesser extent, all of the properties enumerated above, have been proposed, especially for transformers. Among such dielectrics, the "Askarels" have proven to be the most satisfactory and are the most widely used. Same are products resulting from the chlorination of biphenyl or terphenyl and containing from 3 to 7 chlorine atoms; they are most frequently employed in the form of admixtures with one another or with other chlorinated aromatic hydrocarbons, in particular, the trichlorobenzenes and tetrachlorobenzenes. Despite their demonstrated value, these particular dielectrics exhibit the serious disadvantage that they cannot be degraded by biochemical means and cannot easily be degraded by chemical means. This stability of polychlorobiphenyls presents serious hazards from an environmental pollution standpoint, with the result that there is a recognized need and demand for products which are naturally short-lived because they are chemically or biochemically degradable. Products which possess the technical properties referred to above, coupled with good degradability, have not hitherto, been available industrially.
Compare generally U.S. Pat. Nos. 4,019,996, 4,108,789, 4,119,555, 4,142,983 and 4,177,156.